A DSP is the preferable way to increase processor performance for specific applications. DSP applications require a plurality of multiply and multiply-accumulate operations and therefore a Multiplier Accumulator (MAC) unit is added to the traditional micro-processor for enhanced performance.
With the emergence of new applications, like ghost cancellation, wave-form equalization and others, which require various calculations using long filters (referred to also as long filter calculations or operations) system designers started to use conventional DSP's with external memory and known per se external filter devices (referred to occasionally also as "correlators").
The structure and operation of conventional correlator is well known in the literature, see for example "Theory and Application of Digital Signal Processing", Lawrence R. Rabiner- Bernard Gold; Prentice Hill.
The present invention will be described predominantly (but not exclusively) with reference to a ghost cancellation application and accordingly, for a better understanding, there follows a brief explanation of the known per se "ghost" phenomenon in the context of modern television transmissions, as well as conventional techniques for ghost cancellation. In communication applications, the same operation is designated signal equalization, where the received signal is cleaned from channel interferences such as lost of bandwidth and echoes.
Echo signals or ghost signals have been one of the major problems in modern television transmission. When a transmitted signal is received from the air, ghosts can be caused by reflections from mountains, buildings, and the like. In a television signal received through cable, the ghosts can be due to discontinuities of the connectors. Although the ghosts are generated in a radio frequency spectrum, the process in which the ghost is created can be accurately modelled as a linear distortion in a baseband signal. Therefore, cancelling the ghosts can be accomplished by passing the baseband signal through a linear filter which is an inverse of the linear model of the ghosting process.
Typically, ghost cancellation and equalization sequence obey the following algorithmic expression: ##EQU1##
Prior efforts in eliminating ghosts used analog technology such as a charge coupled device (CCD) transversal filter which did not produce significant improvement due to inaccuracy of the filter. More recently, a ghost canceler reference (GCR) signal has been proposed for use in the training of filters. Digital filters have become widely accepted as alternative methods to implement the ghost cancelers.
Ghost signals can arrive at a receiver before and after the main signal, resulting in a precursor and postcursor ghost, respectively. A filter suitable for cancelling the precursor ghost is a physically non-realizable infinite impulse response (IIR) filter, which can be approximated by a longer finite impulse response (FIR) filter. The postcursor ghost can be cancelled by an OR filter. Therefore, a typical digital filter for eliminating ghost signals is an FIR feedforward filter for eliminating precursor ghosts followed by an IIR feedback filter, for eliminating postcursor ghosts.
In order to calculate the filter coefficients, a ghost canceler reference (GCR) signal is transmitted from the broadcasting station, typically in one line of the vertical blanking interval (VBI). The coefficient of the filters can be estimated by comparing the received GCR signal with a stored standard GCR signal.
Having described, in general, the ghost phenomenon and a typical sequence of ghost cancellation, the hitherto known application of a DSP for accomplishing ghost cancellation will now be described.
Accordingly by a specific application, both the DSP and the external filter devices coupled thereto execute similar type long filter operations. By this embodiment, the DSP executes long-correlator operations for calculating coefficient which are then loaded to the external filter. Having been loaded with the coefficients, the filter executes long-convolution calculation for filtering the incoming video data thereby accomplishing the desired ghost cancellation.
This approach of a system consisting of a DSP and an external filter meets, thus, the functional requirement of ghost cancellation but at the same time it has some significant shortcomings.
For one, the system is built out of many devices which necessitates a relatively complex interfacing between the various components which renders the structure of the device cumbersome thereby adversely affecting the filter's capability to operate in changing modes of operation that are prescribed by different applications. Moreover, the device of the kind specified is too expensive for low-cost consumer applications, thereby posing undue constraints insofar as many commercial applications are concerned.